Signal frequency converter



Dec. s, 1960 2,963,660

G. H. TOWNER v SIGNAL FREQUENCY CONVERTER Original Filed April 2, 1954 2Sheets-Sheet 1 Dec. 6, 1960 G. H. 'rowNER v 2,963,660

SIGNAL FREQUENCY CONVERTER Urignal Filed April 2. 1954 2 Sheets-Sheet 2United States Patent" O 2,963,660l Y SIGNAL FREQUENCY CONVERTER GeorgeH. Towner, San Diego, Calif., assigner to NorthropCorporation, acorporation of California Original application Apr. 2, 19'54, Ser. No.420,644,

now Patent No. 2,817,062, filed Dec. 17, 1957. Divided and thisapplication July 12, 1957, Ser. No. 671,639

4 Claims. (Cl. 332-52) This invention relates to frequency conversionmeans and, more particularly, to a frequency converting circuitespecially suited for servo work, and is a division of my priorcopending application Serial No. 420,644, filed April 2, 1954, nowPatent No. 2,817,062, issued December 17, 1957. i

ln electronic art, applications of frequency conversion means aremanifold, as in the operation of low frequency servo motors from anerror signal of a higher frequency, for example, or in the adaptation ofdifferent frequency precision instruments to a central frequency source.Prior means for signal frequency conversion as mixed type converters andordinary modulators and their demodulators, for example, presentedproblems of undersired time lag introduced into the output, circuitcomplexity and poor reliability for use in servo systems.

It is, accordingly, an object of this invention to provide a frequencyconversion circuit that is simple in construction, reliable in operationand which causes a minimum amount of inherent time lag (delay) to aninput signal.

It is a further object of this invention to provide a frequencyconversion circuit which prevents input quadnature signal componentsfrom appearing in the output as quadrature signals of the rnewfrequency.

Briefly, the present invention comprises two diodes connected in seriesacross a center-tapped secondary winding of a transformer. Thewcentertap is connected to ground,` and the junction of the two diodes isresistively connected to an input signalvsource. A first referencevoltage of input signal frequency and a` second reference voltage ofoutput frequency are connected in series with the primary winding of thetransformer. The two diodes serve effectively as switches, theirconduction status being governed by the reference frequencies whichmodulate the input signal. An output signal is derived across aresistance which is serially coupled by a capacitance to the junction ofthe two diodes. This output signal is converted into a sine wave bymeans of a resonant circuit in the plate circuit of an amplifier tubeconnected across the resistance.

Other features and objects of the invention will be more clearlyrecognized from reference to the following specitication andaccompanying drawings in which:

Figure 1 is a circuit diagram of a preferred embodiment of the presentinvention.

Figure 2 is a composite graph showing a series of waveforms whichillustrate circuit response of the invention to a gradually increasinginput signal.

Figure 3 is a graph showing a waveform illustrating circuit output to aconstant amplitude quadrature input signal.

Reference is first made to the circuit diagram of Figure 1. Atransformer 1 has a primary 1a connected in series with a first voltagev1 and a second voltage v2. Voltage v1 is a reference voltage from a 400c.p.s. voltage source, l

2,963,660 Patented Dec. 6, 1960 v1 and v2 are equal in magnitude.Transformer 1 has a secondary winding 1b with a center tap grounded asshown.

An input signal V3 is provided at input terminals 3 and 4, of whichterminal 4 can be connected to ground. Input signal v3 is a 400 c.p.s.signal which can vary in magnitude and is to be converted into a 60c.p.s. output signal. This output signal (v9) is the result of onlythose input signal components which are in phase with the 400 c.p.s.reference frequency, because input quadrature signal components aresuppressed from the output by means of two diodes D1 and D2 connectedeffectively as discriminating switches. The plate of D1 and the cathodeof D2 are connected together to terminal 3 through an isolatingresistance R5. The cathode of D1 is connected through a resistance R1 toone end of transformer secondary 1b, while the plate of D2 is connectedthrough a resistance R2 to the other end of secondary 1b.

The plate of D1 and cathode of D2 are connected to a resistance R6through a coupling capacitance C1. The other end of resistance Re isconnected to ground, and a tap on R6 is directly connected to the gridof a tube T1 as shown. Tube T1 is suitably biased by cathode resistanceR7 and the plate of T1 is connected to B+ through a coil L1 which isshunted by a capacitance C2 to form a resonant 60 c.p.s. circuit. Outputsignal v9 is secured across terminals 5 and 6, terminal 5 beingconnected to the plate of T1 and terminal 6 connected to ground.

In Figure 2, there are shown eight separate graphs having curves plottedon abscissas, of the same time scale. The different graphs are labeledv1, v2, v3, v4, v5, vv6, v7, and v8 corresponding to voltageidentification on Figure l. The first graph labeled v1 shows a constantmagnitude 400 c.p.s. voltage wave and the second graph v2 shows a 60c.p.s. wave of the same magnitude. The two voltages are applied inseries to the primary of transformer 1. The next graph, of v3, shows alinearly iucreasing magnitude input signal in phase with v1 providedacross terminals 3 and 4, this example input signal attaining a constantmagnitude as indicated. With this input signal (v3) impressed acrossterminals 3 and 4, voltage v4 between the common junction of diodes D1and D2 to ground takes shape as given by the fourth graph of v4. Aseries of gradually increasing peaks appear which are cut down orlimited by a 60 c.p.s. envelope of v2. When the magnitude of v3 issmall, the full 400 c.p.s. positive peaks of v4 rare undisturbed aroundthe 60 c.p.s. positive peak points of an envelope of v2 superimposed onv4, but are reduced to zero at the negative peak points of the envelope.Negative peaks due to v3 do not appear in v4 because v1 is in phase withv3 and the negative v1 peaks on diode D1 cathode cause conduction whichshorts out these portions from appearing at v4. Consequently, v5 isshown as a gradually increasing magnitude 60 c.p.s. wave which is theoutput signal provided to, for example, an A.C. servo rnotor. If the 400c.p.s. signal phase of v3 is changed to 180, the graph of v4 turnsupside down. The envelope of v4 produces v5 which, in turn, isessentially v2. v

The sixth graph, labeled v6, is la plot of voltage between the cathodeof D1 and ground. A series of positive 400 c.p.s. peaks, limited by a 60c.p.s. envelope, exists as shown. Similarly, the voltage waveformappearing between the plate of D2 and ground is as shown in thefollowing graph for v7. The waveform v7 is identical to v6 except thatthe 400 c.p.s. peaks are negative and are limited by a 60 c.p.s.envelope below the abscissa. The last graph of Figure 2 is labeled v8and illustrates the superimposition of v1 on v2. This voltage v8 isidentified as the waveform across primary 1a but can represent thewaveform for the transformed voltage across the secondary 1b.

A necessary requirement for a diode to conduct current is that the platethereof be at a sufficiently higher positive voltage than thecorresponding cathode. Careful examination of the circuit diagrams willreveal that this condition occurs during the times that v6 and -vq(Figure 2) are zero (spaces between pulses on the abscissa). Toillustrate how quadrature signals in the input signal can be eliminatedfrom the output, assume that the input signal differs by 90 `degreeswith the reference 400 c.p.s. signal v1 and is of a constant magnitude.Thus, v3 becomes a constant magnitude 400 c.p.s. signal which can leador lag v1 by 90 degrees. The output v4 in Figure 3 is shown for alagging quadrature input signal. A leading input quadrature signal wouldresult in a reversal of the half peaks; a negative chopped peak followeddirectly by a positive chopped peak. The crossover point between thesechopped peak pairs correspond to maximum peak points on the 400 c.p.s.reference signal v1 which are separated by 360 degrees. Whether thecrossover points correspond to the maximum peak points of the positivehalves or the negative halves, depends simply on transformer leadconnections with the 400 c.p.s. signal source. Reversal of leads wouldcause crossover point correspondence to change from one to the othermaximum point halves. From Figure 3, it is to be noted that the averageoutput for 1/60 second or longer is zero, hence the quadraturecomponents are eliminated from the output. Two units of the inventioncould be operated, properly phased, to secure full wave rectification ofinput signal and hence derive a more nearly sinusoidal output to aresonant circuit.

Thus, va simple circuit for frequency conversion, eliminating quadraturecomponents, has been described. It is to be understood, however, thatthe invention is not limited to the specific features shown, but thatthe means and construction herein disclosed comprise the preferred formof putting the invention into effect, and the invention is, therefore,claimed in any of its forms or modifications within the legitimate andvalid scope of the appended claims.

What is claimed is:

l. Frequency conversion means, comprising: an input adapted to receive asignal of an input frequency; a first rectifier having a plate and acathode; a second rectifier having a plate and a cathode; a junctionbetween the plate of said rst rectifier and the cathode of said secondrectifier, and means connecting said junction to one side of said input;output means connected to said junction for a signal of an outputfrequency; a transformer having a primary and a center-tapped secondary;a reference voltage source of said input frequency and a referencevoltage 4 source of said output frequency connected to said primary toenergize the latter; means connecting the cathode of said firstrectifier and the plate of said second rectifier respectively to an endlof said secondary; and means connecting said center tap to the otherside of said input.

2. Apparatus in accordance with claim 1 wherein said output meansincludes tuned circuit means tuned only to said output frequency,whereby a true sine Wave may be produced.

3. Frequency conversion means, comprising: a pair of input terminals forreceiving a signal at an input frequency to be converted to an outputsignal of a desired output frequency different from said inputfrequency; a first rectifier having an anode and a cathode; a secondrectifier having an anode and a cathode, the anode of said firstrectifier connected to the cathode of said second rectifier at a commonjunction point; an input impedance connected from one of said inputterminals to said common junction point; an output circuit from whichthe output signal is taken, connected from said junction point `to thesecond of said input terminals; a transformer having a primary and acenter-tapped secondary; a reference voltage source of input frequencyand a reference voltage source of output frequency connected in serieswith said transformer primary; a first resistance connecting the cathodeof said first rectifier to one end of said transformer secondary; asecond resistance connecting the anode of said second rectifier to theother end of said transformer secondary; and means connecting the centertap of said secondary to said second input terminal.

4. Apparatus in accordance with claim 3 including an amplifieroperatively connected to said output circuit, and a resonant network inthe output fof said amplifier, said network being resonant at saidoutput frequency for obtaining a sine wave output signal.

References Cited in the file of this patent UNITED STATES PATENTS2,244,799 Paddle June 10, 1941 2,304,135 Wise Dec. 8, 1942 2,446,188Miller Aug. 3, 1948 2,486,076 Strutt et al. Oct. 25, 1949 2,608,650Myers Aug. 26, 1952 2,799,829 Gordon et al. July 16, 1957 FOREIGNPATENTS 143,258 Sweden Dec. 15, 1953 2,357,499 Great Britain 1931 m, w:f

